In 1928 Chandrasekhara Raman reported a process in which a material when irradiated with light would simultaneously absorb a photon at a first energy level and emit a photon at a second energy level. The energies of the two photons differed by an amount corresponding to the difference between two quantum-mechanical levels of the medium. Raman scattering, as the phenomenon came to be known, provided a tool for the spectroscopic investigation of energy levels not accessible by the usual absorption and emission techniques. For the first thirty-five years Raman scattering spectroscopy was very laborious, and was important more for the quantum-mechanical principles it illustrated than for its practical applications.
The development of coherent Raman spectroscopy, recently made possible by the availability of high powered tunable lasers, has provided a revolutionary new means for obtaining Raman spectra. Rather than randomly scattering photons as in earlier techniques, a medium can now be irradiated by a coherent light beam tuned over a predetermined frequency spectrum and the Raman modes studied through a beam of coherent radiation emitted from the medium which contains details of the Raman spectrum. Advantages of the coherent Raman techniques result because laser fields at two different frequencies can force a particular Raman mode of a medium to produce an oscillating dielectric constant which then interacts with one of the fields to produce a coherent output beam. The power in this beam can be many orders of magnitude larger than that in spontaneously scattered radiation and spatial filtering can be used to separate the output beam from unwanted radiation.
A recent development in Raman spectroscopy utilized a Kerr effect which is inducted in a scattering medium only at Raman-shifted frequencies. This Raman-induced Kerr effect is obtained when a strong polarized monochromatic pump beam at a frequency w.sub.1 intersects within the medium a weak linearly polarized probe beam at a frequency w.sub.2, thereby inducing complex, anisotropic changes in the refractive indices experienced by the weak probe beam. These changes exhibit resonances when w.sub.1 -w.sub.2 is near a frequency of a Raman-active vibration in the scattering medium. Being anisotropic, the changes cause portions of the weak probe beam to shift polarization by 90 degrees as it passes through the scattering medium. A Glan-Thompson prism is oriented to pass only that portion of the probe beam whose polarization has thus shifted with respect to the original probe beam polarization. The output of the Glan-Thompson prism was typicallly analyzed with a spectrometer. Problems with Raman-induced Kerr effect spectroscopy (RIKES) arise because the intensity of the polarization-shifted probe beam is quadratic in the Raman cross-section of the medium and because there is a background linear in probe laser intensity which results from optical imperfections in sample cell windows, lenses, mirrors, etc., thereby obscuring the Raman modes.
This invention relates to methods and apparatus for greatly enhancing the sensitivity of existing coherent Raman spectroscopy techniques by heterodyning a Raman induced optical field with a stable local oscillator field. The result is a dramatic improvement in signal-to-noise ratio due to the creation of a heterodyned cross-term signal which is proportional to the product of the Raman-induced field and the local oscillator field and therefore linearly proportional to the Raman cross-section of the sample.
An apparatus for heterodyne detection of coherent Raman signals from a Raman active sample includes a pump laser having an output radiation at a frequency w.sub.1 and a probe laser having an output radiation at a predetermined polarization and a frequency w.sub.2, both laser outputs being directed into and intersecting within the sample. The pump laser frequency w.sub.1 is set so that w.sub.1 -w.sub.2 approximates a Raman mode frequency of the sample which, due to a Raman-induced Kerr effect in the sample, causes a portion of the probe laser output to shift polarization by 90 degrees. A polarization analyzer, such as a Glan-Thompson prism, is provided for passing portions of the probe laser output having a predetermined polarization after these portions have passed through the sample. Also included in the apparatus is a means for generating a local oscillator output having substantially the same frequency as that of the probe laser output and a means for heterodyning the local oscillator output with the probe laser output portion which shifted polarization. Means for detecting a heterodyned output portion proportional to the shifted probe laser output and the local oscillator output is also included.
In a first embodiment, the Glan-Thompson prism is oriented so that most of the probe laser output portion whose polarization has shifted by 90 degrees will be passed and some of the unshifted probe laser output will also be passed. The unshifted probe laser output constitutes in effect a local oscillator. The heterodyning of these two signals provides the heterodyned cross-term signal described above.
In a second embodiment a local oscillator output is generated by a retardation plate such as a quarter wave plate or a strained glass window, which is disposed to intersect the probe laser output prior to its entering the Raman-active sample. In this embodiment the Glan-Thompson prism is oriented so that only the polarization shifted portion of the probe beam will be passed. The retardation plate converts the linearly polarized probe laser beam to an elliptically polarized beam a portion of which has the same polarization as that of the probe laser output having the Raman-induced 90 degree polarity shift. This portion constitutes in effect a local oscillator output. Here the Glan-Thompson prism provides the heterodyning means by passing the above two outputs.
In a third embodiment a local oscillator output is obtained by reflecting a portion of the linearly polarized probe laser output prior to its entering the sample and routing it directly to the detector where it is heterodyned with the Glan-Thompson prism output. The reflected probe laser output is passed through a variable phase retarder plate so that its phase relative to the probe laser output can be adjusted.
In conjunction with the above local oscillator generating and heterodyning techniques, the invention also discloses modulation of the pump laser output so that the heterodyned cross-term signal will have predetermined modulation characteristics thereby enhancing isolation of the cross-term signal through signal processing techniques well developed in the art.